TL;DR - Don't use this addon. Use Equalizer Plus instead.

It increases the volume of videos, but there are three flaws with this addon.

The first flaw is that the interface of the addon popup doesn't look very good.

The second flaw is that I think the video has to be playing for the sound adjustments to take effect. I don't think that the sound adjustments work if the video is momentarily paused. But other sound booster extensions don't have this problem and will let you adjust the volume even if the video is paused.

The third flaw is the most serious. If you adjust the sound using this extension, the fullscreen button in the video (F11 still works) will quit working. 99% of all sound booster addons have this same exact problem. But I found an extension that lets you do both - increase the sound volume AND use the fullscreen button. It's called Equalizer Plus. Someone also suggested another addon that also lets you increase the sound volume AND use the fullscreen button, but I didn't try it and the interface looks bad so I don't want to try it. But there you go. There are at least one or two addons out there that let you use the video's fullscreen button (which I prefer over using the F11 hotkey) after you've made sound adjustments. I highly recommend Equalizer Plus. The interface looks better too.

Actually, there is a fourth flaw. There have been reports that this addon has ads/tracking and malicious code. One guy on GitHub actually took this extension, removed the ads/tracking and made a new version of the addon with the same name, which you can find on GitHub. But I recommend not using either version and using Equalizer Plus instead.